Not Applicable.
Not Applicable.
1. Field of the Invention
The present invention relates to electrohydraulic systems for operating machinery, and in particular to control algorithms for such systems.
2. Description of the Related Art
A wide variety of machines have moveable members which are operated by an hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve. Traditionally the hydraulic valve was manually operated by the machine operator. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid operated valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located near an operator station, but can be located adjacent the actuator being controlled. This change in technology also facilitates sophisticated computerized control of the machine functions.
Application of pressurized hydraulic fluid from a pump to the actuator can be controlled by a proportional solenoid operated spool valve that is well known for controlling the flow of hydraulic fluid. Such a valve employs an electromagnetic coil which moves an armature connected to the spool that controls the flow of fluid through the valve. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. Either the armature or the spool is spring loaded to close the valve when electric current is removed from the solenoid coil. Alternatively a second electromagnetic coil and armature is provided to move the spool in the opposite direction.
When an operator desires to move a member on the machine a joystick is operated to produce an electrical signal indicative of the direction and desired rate at which the corresponding hydraulic actuator is to move. The faster the actuator is desired to move the farther the joystick is moved from its neutral position. A control circuit receives a joystick signal and responds by producing a signal to open the associated valve. A solenoid moves the spool valve to supply pressurized fluid through an inlet orifice to the cylinder chamber on one side of the piston and to allow fluid being forced from the opposite cylinder chamber to drain through an outlet orifice to a reservoir, or tank. A hydromechanical pressure compensator maintains a nominal pressure (margin) across the inlet orifice portion of the spool valve. By varying the degree to which the inlet orifice is opened (i.e. by changing its valve coefficient), the rate of flow into the cylinder chamber can be varied, thereby moving the piston at proportionally different speeds. Thus prior control methods were based primarily on inlet orifice metering using an external hydromechanical pressure compensator.
Recently a set of proportional solenoid operated pilot valves has been developed to control fluid flow to and from the hydraulic actuator, as described in U.S. Pat. No. 5,878,647. In these valves, the solenoid armature acts on a pilot poppet that controls the flow of fluid through a pilot passage in a main valve poppet. The armature is spring loaded to close the valve when electric current is removed from the solenoid coil.
The control of an entire machine, such as an agricultural tractor or construction equipment is complicated by the need to control multiple functions simultaneously. For example, in order to operate a back hoe, hydraulic actuators for the boom, arm, bucket, and swing have to be simultaneously controlled. The loads acting on each of those machine members often are significantly different so that their respective actuators require hydraulic fluid at different pressures. The pump often is a fixed displacement type with the outlet pressure being controlled by an unloader. Therefore, the unloader needs to be controlled in response to the function requiring the greatest pressure for its actuator. In some cases the pump may be incapable of supplying enough hydraulic fluid for all of the simultaneously operating functions. At those times it is desirable that the control system allocate the available hydraulic fluid among those functions in an equitable manner, taking into account that some function may deserve fluid on a higher priority than other functions.
A branch of a hydraulic system has a hydraulic actuator connected between a supply line containing pressurized fluid and a return line connected to a tank. The method for operating the hydraulic system comprises requesting a desired velocity for the hydraulic actuator. Such a request may emanate from an operator input device for the machine on which the hydraulic circuit is a component. A parameter, which varies with changes of a force acting on the hydraulic actuator, is sensed to provide an indication of that force. For example, this parameter may be pressure at the hydraulic actuator which indicates the load on the hydraulic actuator.
An equivalent flow coefficient, characterizing the fluid flow through the hydraulic system branch that is required to achieve the desired velocity, is derived based on the desired velocity and the sensed parameter. Fluid flow and/or pressure in the hydraulic system can be controlled based on the equivalent flow coefficient. For example, valves in the system are opened to a degree that is determined from the equivalent flow coefficient in order to operate the hydraulic actuator at the desired velocity.
Another hydraulic circuit branch, with which the present method can be used, has an assembly of four electrohydraulic proportional valves. A first one of these valves couples a first port of a hydraulic actuator, such as a double acting hydraulic cylinder, to the supply line containing pressurized fluid. A second electrohydraulic proportional valve couples a second port of the hydraulic actuator to the supply line, a third one of these valves is between the first port and a return line connected to a tank, and the fourth valve couples the second port to the return line. In this arrangement, activation of selected pairs of the four electrohydraulic proportional valves enables operation of the hydraulic actuator in several metering modes, which include powered extension, powered retraction, high side regeneration, and low side regeneration. In each metering mode, measurements of pressures at the ports of the hydraulic actuator and in the supply and return lines, as well as physical characteristics of the hydraulic actuator, are used along with the desired velocity to derive a valve flow coefficient for each electrohydraulic proportional valve which is to open in the selected mode. The respective valve flow coefficients then are used to determine the degree to which to open those valves in order to drive the hydraulic actuator at the desired velocity.
Another aspect of the present invention is using the equivalent flow coefficient for the hydraulic circuit branch to regulate pressure in the supply and return lines to properly drive the hydraulic actuator.